(1) Field of the Invention
The present invention relates to an image forming apparatus such as a printer or copier provided with a fixing device, and especially to a technology for reducing the paper jam from occurring to a recording sheet having passed through a fixing nip.
(2) Description of the Related Art
An image forming apparatus such as a printer or copier is provided with a fixing device for thermally fixing a toner image (unfixed image) formed on a recording sheet. The fixing device typically includes a fixing rotator, which is a fixing roller or a fixing belt, and a pressing roller. The fixing device causes a recording sheet to pass through a fixing nip which is formed by the fixing rotator and the pressing roller pressing the fixing rotator so that the toner image is thermally fixed onto the recording sheet.
The fixing device is also provided with a separator for separating the recording sheet from the fixing rotator when the recording sheet comes out of the fixing nip after the thermal fixing. In the case where the separator is a sheet separating claw, the tip of the sheet separating claw is located so as to be in contact with the outer surface of the fixing rotator (see Japanese Patent Application Publication No. 2008-139445).
With the above-described structure, when the recording sheet comes out of the fixing nip after a toner image is thermally fixed thereon by passing through the fixing nip, the recording sheet is separated from the fixing rotator by the tip of the sheet separating claw. This prevents occurrence of a paper jam which would occur when the recording sheet having passed through the fixing nip is stuck on the outer circumferential surface of the fixing rotator and does not separate from the fixing rotator.
Meanwhile, in recent years, many recording sheets used in the image forming apparatus contain a large amount of (for example, 10 to 20%) additives such as calcium carbonate to improve the whiteness, smoothness and the like of the sheets. These additives have the property of being likely to attach to the outer circumferential surface of the fixing rotator as the paper dust by the electrostatic phenomenon when they contact with the fixing rotator during the thermal fixing. When such paper dust is attached to the outer circumferential surface of the fixing rotator, the toner may further be attached to the paper dust on the surface. In such a state, when a recording sheet on which a toner image has been formed, passes through the fixing nip, the toner attached to the paper dust sticks to the toner of the recording sheet and the recording sheet is strongly stuck to the outer circumferential surface of the fixing rotator.
When the recording sheet is strongly stuck to the outer circumferential surface of the fixing rotator, it become difficult for the tip of the sheet separating claw to separate the recording sheet from the outer circumferential surface of the fixing rotator, and as a result, the recording sheet is wound around the outer circumferential surface of the fixing rotator, causing a paper jam to occur.
One might think that a cleaning device may be provided to remove the paper dust from the fixing rotator. However, this causes a deficiency that, when provided, the cleaning device removes heat partially from the fixing rotator, thereby decreasing the heating efficiency of the fixing rotator during the warm-up and increasing the time required for the warm-up.